My 323 had this misfiring problems too... But i've solved it.I did re-timing,changed suitable spark plugs,cleaned throttle body,air cleaner replaced,most importantly FUEL FILTER,changed engine oil... Now my car runs like new...

 

Okay I might be a little Biased.. as a Mazda Tech, I totally agree with your mazda dealer.. Change the coils.

 

alrighty, I changed the coils today. Drove fine for about 15 miles and now my CEL is on again. GRRRRRRR.

 

What about the plug wires, plug caps or springs? Is the code the same as before?
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yes, the code is the same. It started with 300 now the code is 421. I've changed the plugs, wires, o2 sensor, coil packs.

 

It could be your PCM (Powertrain Control Module), you know, the computer. Take it to the dealer.
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